Positron emission tomography (PET) is a nuclear medicine technique which allows us to obtain some anatomic data (e.g., axial, coronal or sagittal images of the brain) as well as dynamic functional data (such as regional cerebral glucose consumption rate). The unique property of PET is that it provides physiologic and pathophysiologic information not available with any other imaging procedure. Using a variety of radiopharmaceuticals as tracers, we have investigated with PET, brain tumors, movement disorders (Parkinson's disease, in particular), the dementias, narcolepsy and cerebral involvement in AIDS. New information has been gathered, both in the basic and in the clinical (patient management) areas.